Catalyst and electrode catalyst layer, membrane electrode assembly, and fuel cell using the catalyst

ABSTRACT

[Object] Provided is a catalyst having excellent gas transportability. 
     [Solving Means] Disclosed is a catalyst comprising a catalyst support and a catalyst metal supported on the catalyst support, wherein the catalyst includes pores having a radius of less than 1 nm and pores having a radius of 1 nm or more, wherein a pore volume of the pores having a radius of less than 1 nm is 0.3 cc/g support or more or a mode radius of a pore distribution of the pores having a radius of less than 1 nm is 0.3 nm or more and less than 1 nm, and wherein the catalyst metal is supported inside the pores having a radius of 1 nm or more.

TECHNICAL FIELD

The present invention relates to a catalyst, particularly an electrodecatalyst used for a fuel cell (PEFC) and an electrode catalyst layer, amembrane electrode assembly, and a fuel cell using the catalyst.

BACKGROUND ART

A polymer electrolyte fuel cell using a proton conductive solid polymermembrane operates at a low temperature in comparison to other types offuel cells, for example, a solid oxide fuel cell or a molten carbonatefuel cell. For this reason, the polymer electrolyte fuel cell has beenexpected to be used as a power source for energy storage system or adriving power source for a vehicle such as a car, and practical usesthereof have been started.

In general, such a polymer electrolyte fuel cell uses expensive metalcatalyst represented by platinum (Pt) or a Pt alloy, which leads to highcost of the fuel cell. Therefore, development of techniques capable oflowering the cost of the fuel cell by reducing a used amount of noblemetal catalyst has been required.

For example, Patent Literature 1 discloses an electrode catalyst havingcatalyst metal particles supported on a conductive support, wherein anaverage particle diameter of the catalyst metal particles is larger thanan average pore diameter of fine pores of the conductive supports. ThePatent Literature 1 discloses that, according to the above-describedconfiguration, the catalyst particles are not allowed to enter the finepores of the supports, so as to increase a ratio of the catalyst metalparticles used in a three phase boundary, and thus, to improve useefficiency of expensive noble metal.

CITATION LIST Patent Literature

Patent Literature 1: JP-A-2007-250274 (US 2009/0047559 A1)

SUMMARY OF INVENTION

However, the catalyst disclosed in the Patent Literature 1 has problemsin that the electrolyte and the catalyst metal particles are in contactwith each other, so that the catalyst activity decreases. On the otherhand, if the catalyst metals are supported inside fine pores which theelectrolyte cannot enter so as for the electrolyte and the catalystmetal particles not to be in contact with each other, a transportingdistance of a gas such as oxygen is increased, and thus, a gastransportability is deteriorated, a sufficient catalyst activity cannotbe exhibited, and the catalyst performance is deteriorated under highload conditions.

The present invention has been made in light of the aforementionedcircumstances and aims at providing a catalyst having excellent catalystactivity and excellent gas transportability.

Another object of the present invention is to provide an electrodecatalyst layer, a membrane electrode assembly, and a fuel cell having anexcellent power generation performance.

The present inventors have intensively studied to solve theaforementioned problems, to find that the problems can be solved by acatalyst having a specific pore distribution, and eventually the presentinvention has been completed.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic cross-sectional diagram illustrating a basicconfiguration of a polymer electrolyte fuel cell according to anembodiment of the present invention.

FIG. 2 is a schematic cross-sectional diagram illustrating a shape and astructure of a catalyst according to an embodiment of the presentinvention.

FIG. 3 is a schematic diagram illustrating a relationship between acatalyst and an electrolyte in a catalyst layer according to anembodiment of the present invention.

FIG. 4 is a diagram illustrating pore distributions of comparativecatalysts F to H of Comparative Examples 1 to 3.

DESCRIPTION OF EMBODIMENTS

A catalyst (in this description, also referred to as an “electrodecatalyst”) of the present invention is configured to comprise a catalystsupport and a catalyst metal supported on the catalyst support. Herein,the catalyst satisfies the following features (a) to (c):

(a) the catalyst contains pores (primary pores) having a radius of lessthan 1 nm and pores (primary pores) having a radius of 1 nm or more;

(b) a pore volume of the pores having a radius of less than 1 nm is 0.3cc/g support or more; and

(c) the catalyst metal is supported inside the pores having a radius of1 nm or more.

Also, a catalyst of the present invention is configured to comprise acatalyst support and a catalyst metal supported on the catalyst support.Herein, the catalyst satisfies the following features (a), (d) and (c):

(a) the catalyst contains pores having a radius of less than 1 nm andpores having a radius of 1 nm or more;

(d) a mode radius of a pore distribution of the pores having a radius ofless than 1 nm is 0.3 nm or more and less than 1 nm; and

(c) the catalyst metal is supported inside the pores having a radius of1 nm or more.

According to the catalyst having the above-described features, thecatalyst metal is placed in a relatively large pore so as to suppressthe contact with the electrolyte, and a gas transport path is secured byrelatively small pores so as to improve gas transportability. As aresult, a catalyst having excellent catalyst activity can be provided.In this description, a pore having a radius of less than 1 nm isreferred to as “micropore”. Also, in this description, a pore having aradius of 1 nm or more is referred to as “mesopore”.

The present inventors have found that, in the catalyst disclosed in thePatent Literature 1, since the electrolyte (electrolyte polymer) iseasily adsorbed on the surface of the catalyst in comparison with a gassuch as oxygen, if the catalyst metals are in contact with theelectrolyte (electrolyte polymer), a reaction active area of the surfaceof the catalyst is decreased. On the contrary, the present inventorshave found that, even in the case where the catalyst is not in contactwith the electrolyte, a the catalyst can be effectively used by formingthree phase boundary with water. Therefore, the catalytic activity canbe improved by taking the feature (c) where the catalyst metals aresupported inside the mesopores which the electrolyte cannot enter.

On the other hand, in the case where the catalyst metals are supportedinside the mesopores which the electrolyte cannot enter, since atransporting distance of a gas such as oxygen is increased, a gastransportability is deteriorated, and thus sufficient catalytic activitycannot be exhibited, and the catalyst performance is deteriorated underhigh load conditions. On the contrary, by the feature (b) where asufficient pore volume of micropores which the catalyst metals cannotalmost or never enter is secured, by the feature (d) where a modediameter of micropores is set to be large, sufficient gas transport pathcan be attained. Therefore, a gas such as oxygen can be efficientlytransported to the catalyst metals inside the mesopores, and namely, gastransport resistance can be reduced. By the feature, a gas (for example,oxygen) can pass through the micropores (gas transportability can beimproved), and can be efficiently in contact with the catalyst.

Therefore, according to the present invention, since the microporesexist with a large volume, a reaction gas can be transported through themicropores (paths) to a surface of the catalyst metal(s) existing in themesopores, and thus, gas transport resistance becomes small.Accordingly, the catalyst of the present invention can exhibit highcatalyst activity, and namely, the catalyst reaction can be facilitated.For this reason, a membrane electrode assembly and a fuel cell includingthe catalyst layer using the catalyst according to the present inventionhave an excellent power generation performance.

Hereinafter, embodiments of a catalyst according to the presentinvention and embodiments of a catalyst layer, and a membrane electrodeassembly (MEA) and a fuel cell using the catalyst will be described indetail appropriately with reference to the drawings. However, thepresent invention is not limited to the following embodiments. Inaddition, figures may be expressed in an exaggerated manner for theconvenience of description, and in the figures, scaling factors ofcomponents may be different from actual values thereof. In addition, inthe description of the embodiments of the present invention withreference to the drawings, the same components are denoted by the samereference numerals, and redundant description is omitted.

In this description, “X to Y” representing a range denotes “X or moreand Y or less”, and “weight” and “mass”, “wt % and “mass %” “parts byweight”, and “parts by mass” are used interchangeably. Unless otherwisenoted, operation and the measurement of physical properties areperformed at a room temperature (20 to 25° C.) and a relative humidityof 40 to 50%.

[Fuel Cell]

A fuel cell comprises a membrane electrode assembly (MEA) and a pair ofseparators including an anode-side separator having a fuel gas passagethrough which a fuel gas flows and a cathode-side separator having anoxidant gas passage through which an oxidant gas flows. The fuel cellaccording to the present embodiment has excellent durability and canexhibit a high power generation performance.

FIG. 1 is a schematic diagram illustrating a basic configuration of apolymer electrolyte fuel cell (PEFC) 1 according to an embodiment of thepresent invention. First, a PEFC 1 is configured to comprise a solidpolymer electrolyte membrane 2 and a pair of catalyst layers (anodecatalyst layer 3 a and cathode catalyst layer 3 c) interposing the solidpolymer electrolyte membrane 2. A stacked body of the solid polymerelectrolyte membrane 2 and the catalyst layers (3 a, 3 c) is sandwichedby a pair of gas diffusion layers (GDLs) (anode gas diffusion layer 4 aand cathode gas diffusion layer 4 c). In this manner, the solid polymerelectrolyte membrane 2, a pair of the catalyst layers (3 a, 3 c), and apair of gas diffusion layers (4 a, 4 c) in the stacked state constitutea membrane electrode assembly (MEA) 10.

In the PEFC 1, the MEA 10 is sandwiched by a pair of separators (anodeseparator 5 a and cathode separator 5 c). In FIG. 1, the separators (5a, 5 c) are illustrated to be positioned at two ends of the MEA 10illustrated. In general, in a fuel cell stack where a plurality of MEAsare stacked, the separator is also used as a separator for adjacent PEFC(not shown). In other words, MEAs in a fuel cell stack are sequentiallystacked through the separator to constitute the stack. In an actual fuelcell stack, a gas sealing member is disposed between the separators (5a, 5 c) and the solid polymer electrolyte membrane 2 and between thePEFC 1 and a different PEFC adjacent thereto. However, it is omitted inFIG. 1.

The separators (5 a, 5 c) are obtained by applying a pressing process toa thin board having a thickness of, for example, 0.5 mm or less to forma corrugating shape illustrated in FIG. 1. Convex portions of theseparators 5 a and 5 c seen from the MEA side are in contact with theMEA 10. This secures an electrical connection with the MEA 10. Concaveportions (spaces between the separator and the MEA formed by thecorrugating shapes of the separators) of the separators (5 a and 5 c)seen from the MEA side function as a gas passage for passing a gasduring the operation of the PEFC 1. Specifically, a fuel gas (forexample, hydrogen) flows through a gas passage 6 a of the anodeseparator 5 a, and an oxidant gas (for example, air) flows through a gaspassage 6 c of the cathode separator 5 c.

On the other hand, concave portions of the separators (5 a, 5 c) seenfrom the side opposite to the MEA side function as a coolant passage 7for passing a coolant (e.g. water) for cooling the PEFC during theoperation of the PEFC 1. In addition, manifolds (not shown) aretypically installed in the separators. The manifold functions as aconnecting means for connecting cells when the stack is configured.According to the configuration, a mechanical strength of the fuel cellstack can be secured.

In the embodiment illustrated in FIG. 1, each of the separators (5 a, 5c) is formed in a corrugating shape. However, the separator is notlimited to such a corrugating shape. If it can serve as a gas passageand a coolant passage, arbitrary shape such as a flat shape and apartially corrugating shape may be employed.

The fuel cell including the MEA according to the present invention asdescribed above has excellent performance of power generation. Herein,the type of the fuel cell is not particularly limited. In the abovedescription, the polymer electrolyte fuel cell is exemplified, butbesides, an alkali fuel cell, a direct methanol fuel cell, a micro fuelcell, and the like may be exemplified. Among the fuel cells, due to asmall size and capability of obtaining high density and high power, apolymer electrolyte fuel cell (PEFC) is preferred. In addition, the fuelcell is useful as a power source for energy storage system besides apower source for a vehicle such as a car where amounting space islimited. Among the power sources, the fuel cell is particularlypreferably used as a power source for a vehicle such as a car where ahigh output voltage is required after the stopping of operation for arelatively long time.

A fuel used for operating the fuel cell is not particularly limited. Forexample, hydrogen, methanol, ethanol, 1-propanol, 2-propanol, 1-butanol,secondary butanol, tertiary butanol, dimethyl ether, diethyl ether,ethylene glycol, diethylene glycol, or the like can be used. Among them,in view of capability of high output, hydrogen or methanol is preferablyused.

In addition, although application use of the fuel cell is notparticularly limited, the fuel cell is preferably applied to vehicles.The electrolyte membrane-electrode assembly according to the presentinvention has excellent power generation performance and durability, andcan be downsized. Therefore, in terms of mountability on a vehicle, thefuel cell according to the present invention is particularlyadvantageous in the case where the fuel cell is applied to a vehicle.

Hereinafter, members constituting the fuel cell according to the presentinvention will be described in brief, but the scope of the presentinvention is not limited only to the following forms.

[Catalyst (Electrode Catalyst)]

FIG. 2 is a schematic cross-sectional diagram illustrating a shape and astructure of a catalyst according to an embodiment of the presentinvention. As illustrated in FIG. 2, a catalyst 20 according to thepresent invention is configured to comprise catalyst metals 22 and acatalyst support 23. The catalyst 20 has pores (micropores) 25 having aradius of less than 1 nm and pores (mesopores) 24 having a radius of 1nm or more. The catalyst metal(s) 22 is supported inside the mesopore24. In addition, at least a portion of the catalyst metals 22 may besupported inside the mesopore 24, and other portions thereof may besupported on the surface of the support 23. However, in terms ofpreventing the contact of the electrolyte with the catalyst metal,substantially all the catalyst metals 22 are preferably supported insidethe mesopores 24. As used herein, the expression “substantially all thecatalyst metals” is not particularly limited if an amount which canimprove a sufficient catalytic activity can be attained. The amount of“substantially all the catalyst metals” is preferably 50 wt % or more(upper limit: 100 wt %), more preferably 80 wt % or more (upper limit:100 wt %), with respect to all the catalyst metals.

In this description, the state “the catalyst metals are supported insidethe mesopores” can be confirmed by a decrease in volume of mesoporesbefore and after the supporting of catalyst metals on a catalystsupport. Specifically, a catalyst support (hereinafter, referred to assimply “support”) contains micropores and mesopores, and the pores havethe respective certain volumes. If catalyst metals are supported in thepore(s), the volumes of the pores are decreased. Therefore, the casewhere a difference between a volume of mesopores of a catalyst (support)before the supporting of catalyst metals and a volume of mesopores of acatalyst (support) after the supporting of catalyst metals [=(volumebefore supporting)−(volume after supporting)] exceeds 0 indicates that“the catalyst metals are supported inside the mesopore(s)”. Similarly,the case where a difference between a volume of micropores of a catalyst(support) before the supporting of catalyst metals and a volume ofmicropores of a catalyst (support) after the supporting of catalystmetals [=(volume before supporting)−(volume after supporting)] exceeds 0indicates that “the catalyst metals are supported inside themicropore(s)”. Preferably, a larger number of catalyst metals aresupported in mesopores than in micropores (namely, (decreased volume ofmesopores before and after the supporting)>(decreased volume ofmicropores before and after the supporting)). By this, gas transportresistance can be reduced and a path for gas transportation can besufficiently secured. In terms of reduced gas transport resistance andsecuring of a path for gas transportation, a decreased pore volume ofmesopores before and after the supporting of the catalyst metals ispreferably 0.02 cc/g or more, more preferably in the range of 0.02 to0.21 cc/g.

A pore volume of pores (micropores) having a radius of less than 1 nm(of a catalyst after catalyst metal(s) is supported) is 0.3 cc/g supportor more, and/or a mode radius of a pore distribution of micropores (of acatalyst after catalyst metal(s) is supported) (maximum frequencydiameter) is 0.3 nm or more and less than 1 nm. Preferably, the porevolume of micropores is 0.3 cc/g support or more, and the mode radius ofthe pore distribution of micropores is 0.3 nm or more and less than 1nm. If the pore volume of micropores and/or the mode diameter are withinsuch ranges, enough micropores for gas transportation can be secured, sothat gas transport resistance becomes small. Therefore, since asufficient amount of a gas can be transported to a surface(s) ofcatalyst metal(s) existing in the mesopores via micropores (path), thecatalyst according to the present invention can exhibit a high catalystactivity, and namely, the catalyst reaction can be facilitated. Inaddition, an electrolyte (ionomer) or liquid (for example, water) cannotenter the micropores, and only a gas can selectively pass through themicropores (gas transport resistance can be reduced). In terms ofeffects of improving gas transportability, the pore volume of microporesis more preferably in the range of 0.3 to 2 cc/g support, particularlypreferably in the range of 0.4 to 1.5 cc/g support. The mode radius ofthe pore distribution of micropores is more preferably in the range of0.4 to 1 nm, particularly in the range of 0.4 to 0.8 nm. In addition, inthis description, the pore volume of pores having a radius of less than1 nm is simply referred to as a “pore volume of micropores”. Similarly,in this description, the mode radius of the pore distribution ofmicropores is simply referred to as a “mode diameter of micropores”.

A pore volume of the pores (mesopores) having a radius of 1 nm or more(of a catalyst after catalyst metal(s) is supported) is not particularlylimited, but it is preferably 0.4 cc/g support or more, more preferablyin the range of 0.4 to 3 cc/g support, particularly preferably in therange of 0.4 to 1.5 cc/g support. If the pore volume is within such arange, a larger number of catalyst metals can be placed (supported) inthe mesopores, and thus, an electrolyte and a catalyst metal(s) in thecatalyst layer are physically separated from each other (contact of acatalyst metal(s) and an electrolyte can be more effectively suppressedand prevented). Therefore, activity of the catalyst metals can be moreeffectively used. In addition, due to existence of a large number ofmesopores, the function and effects by the present invention can befurther remarkably exhibited, so that a catalyst reaction can be moreeffectively facilitated. Also, the micropores function as a gastransport path, and thus, three phase boundary with water can be moreremarkably formed, so that the catalytic activity can be more improved.In this description, the pore volume of pores having a radius of 1 nm ormore is also simple referred to as a “pore volume of mesopores”.

A mode radius (maximum frequent diameter) of a pore distribution ofpores (mesopores) having a radius of 1 nm or more (of a catalyst aftercatalyst metal(s) is supported) is not particularly limited, but it ispreferably in the range of 1 to 5 nm, more preferably in the range of 1to 4 nm, particularly preferably in the range of 1 to 3 nm. If the modediameter of the pore distribution of mesopores is within such a range, asufficient number of catalyst metals can be placed (supported) in themesopores, and thus, an electrolyte and a catalyst metal(s) in thecatalyst layer are physically separated from each other (contact of acatalyst metal(s) and an electrolyte can be more effectively suppressedand prevented). Therefore, activity of the catalyst metals can be moreeffectively used. In addition, due to existence of a large volume ofmesopores, the function and effects by the present invention can befurther remarkably exhibited, so that a catalyst reaction can be moreeffectively facilitated. Also, the micropores function as a gastransport path, and thus, three phase boundary with water can be moreremarkably formed, so that the catalytic activity can be more improved.In this description, the mode radius of the pore distribution ofmesopores is also simply referred to as a “mode diameter of themesopores”.

A BET specific surface area (of a catalyst after catalyst metal(s) issupported) [BET specific surface area of catalyst per 1 g of support(m²/g support)] is not particularly limited, but is 1000 m²/g support ormore, more preferably in the range of 1000 to 3000 m²/g support,particularly preferably in the range of 1100 to 1800 m²/g support. Ifthe specific surface area is within the above-described range, sincesufficient mesopores and micropores can be secured, enough micropores totransport a gas (lower gas transport resistance) can be secured, and alarger number of the catalyst metals can be placed (supported) in themesopores. In addition, an electrolyte and catalyst metals in thecatalyst layer can be physically separated from each other (contactbetween catalyst metals and an electrolyte can be more effectivelysuppressed and prevented). Therefore, activity of the catalyst metalscan be more effectively used. Also, due to existence of a large numberof the micropores and mesopores, the function and effects by the presentinvention can be further remarkably exhibited, so that a catalystreaction can be more effectively facilitated. In addition, themicropores function as a gas transport path, and thus, a three-phaseboundary with water is more remarkably formed, so that the catalyticactivity can be more improved.

In this description, the “BET specific surface area (m²/g support)” ismeasured by a nitrogen adsorption method. Specifically, about 0.04 to0.07 g of a catalyst powder is accurately weighed and sealed in a sampletube. The sample tube is preliminarily dried in a vacuum drier at 90° C.for several hours, to obtain a sample for measurement. For the weighing,an electronic balance (AW220) produced by Shimadzu Co., Ltd. is used. Inthe case of a coated sheet, about 0.03 to 0.04 g of a net weight of acoat layer obtained by subtracting a weight of Teflon (registeredtrademark) (substrate) having the same area from a total weight of thecoated sheet is used as a sample weight. Next, under the followingmeasurement condition, a BET specific surface area is measured. In anadsorption side of adsorption and desorption isotherms, a BET plot isproduced from a relative pressure (P/P0) range of about 0.00 to 0.45,and a surface area and a BET specific surface area are calculated fromthe slope and the intercept.

[Chem. 1]

<Measurement Conditions>

Measurement Apparatus: BELSORP 36, High-Precise Automatic Gas AdsorptionApparatus produced by BEL Japan, Inc.

Adsorption Gas: N₂

Dead Volume Measurement Gas: He

Adsorption Temperature: 77 K (Liquid Nitrogen Temperature)

Measurement Preparation: Vacuum Dried at 90° C. for several hours (AfterHe Purging, Set on Measurement Stage)

Measurement Mode: Adsorption Process and Desorption Process in Isotherm

Measurement Relative Pressure P/P₀: about 0 to 0.99

Equilibrium Setting Time: 180 sec for 1 relative pressure

The “pore radius (nm) of micropores” denotes a radius of pores measuredby a nitrogen adsorption method (MP method). In addition, the “moderadius (nm) of a pore distribution of micropores” denotes a pore radiusat a point taking a peak value (maximum frequency) in a differentialpore distribution curve obtained by a nitrogen adsorption method (MPmethod). Herein, a lower limit of the pore radius of micropores is alower limit value which can be measured by the nitrogen adsorptionmethod, that is, 0.42 nm or more. Similarly, the “pore radius (nm) ofmesopores” denotes a radius of pores measured by a nitrogen adsorptionmethod (DH method). In addition, the “mode radius (nm) of a poredistribution of mesopores” denotes a pore radius at a point taking apeak value (maximum frequency) in a differential pore distribution curveobtained by a nitrogen adsorption method (DH method). Herein, an upperlimit of the pore radius of mesopores is not particularly limited, butit is 5 nm or less.

The “pore volume of micropores” denotes a total volume of microporeshaving a radius of less than 1 nm existing in a catalyst, and isexpressed by volume per 1 g of support (cc/g support). The “pore volume(cc/g support) of micropores” is calculated as an area (integrationvalue) under a differential pore distribution curve obtained accordingto a nitrogen adsorption method (MP method). Similarly, the “pore volumeof mesopores” denotes a total volume of mesopores having a radius of 1nm or more existing in a catalyst, and is expressed by volume per 1 g ofsupport (cc/g support). The “pore volume (cc/g support) of mesopores” iscalculated as an area (integration value) under a differential poredistribution curve obtained according to a nitrogen adsorption method(DH method).

The “differential pore distribution” is a distribution curve obtained byplotting a pore diameter in the horizontal axis and a pore volumecorresponding to the pore diameter in a catalyst in the vertical axis.Namely, when a pore volume of a catalyst obtained by a nitrogenadsorption method (MP method incase of the micropores; and DH method incase of the mesopores) is denoted by V and a pore diameter is denoted byD, a value (dV/d (log D)) is obtained by dividing the differential porevolume dV by a differential logarithm d(log D) of the pore diameter.Next, a differential pore distribution curve is obtained by plotting thedV/d(log D) for an average pore diameter in each section. A differentialpore volume dV denotes an increment of pore volume between measurementpoints.

A method for measuring a radius and a pore volume of micropores by anitrogen adsorption method (MP method) is not particularly limited. Forexample, methods disclosed in well-down literatures such as “Science ofAdsorption” (second edition written by Kondo Seiichi, Ishikawa Tatsuo,and Abe Ikuo, Maruzen Co., Ltd.), “Fuel Cell Analysis Method” (compiledby Takasu Yoshio, Yoshitake Yu, and Ishihara Tatsumi of KAGAKU DOJIN),and an article written by R. Sh. Mikhail, S. Brunauer, and E. E. Bodorin J. Colloid Interface Sci., 26, 45 (1968) may be employed. In thisdescription, the radius and pore volume of micropores by a nitrogenadsorption method (MP method) are a value measured by the methoddisclosed in the article written by R. Sh. Mikhail, S. Brunauer, and E.E. Bodor in J. Colloid Interface Sci., 26, 45 (1968).

A method for measuring a radius and a pore volume of mesopores by anitrogen adsorption method (DH method) is not particularly limited. Forexample, methods disclosed in well-known literatures such as “Science ofAdsorption” (second edition written by Kondo Seiichi, Ishikawa Tatsuo,and Abe Ikuo, Maruzen Co., Ltd.), “Fuel Cell Analysis Method” (compiledby Takasu Yoshio, Yoshitake Yu, and Ishihara Tatsumi of KAGAKU DOJIN),and an article by D. Dollion and G. R. Heal in J. Appl. Chem. 14, 109(1964) may be employed. In this description, the radius and pore volumeof mesopores by a nitrogen adsorption method (DH method) are a valuemeasured by the method disclosed in the article written by D. Dollionand G. R. Heal in J. Appl. Chem. 14, 109 (1964).

A method of manufacturing a catalyst having a specific pore distributionas described above is not particularly limited, but it is important tomake a pore distribution (micropores and mesopores) of a supporttypically the above-described pore distribution. Specifically, as amethod of manufacturing a support having micropores and mesopores andhaving a pore volume of micropores of 0.3 cc/g support or more, themethods disclosed in JP-A-2010-208887 (US 2011/318254 A1, the samehereinafter), WO 2009/75264 (US 2011/058308 A1, the same hereinafter),or the like are preferably used. In addition, as a method ofmanufacturing a support having micropores and mesopores and having amode radius of a pore distribution of micropores of 0.3 nm or more andless than 1 nm, the methods disclosed in JP-A-2010-208887, WO2009/75264, or the like are preferably used.

A material of the support is not particularly limited if pores (primarypores) having above-described pore volume or mode radius can be formedinside the support and if the support has enough specific surface areaand enough electron conductivity to support a catalyst component insidethe mesopores in a dispersed state. Preferably, a main component iscarbon. Specifically, carbon particles made of carbon black (KetjenBlack, oil furnace black, channel black, lamp black, thermal black,acetylene black, or the like), activated charcoal, or the like may beexemplified. The expression “main component is carbon” denotes that thesupport contains carbon atoms as a main component, and includes both ofthe configurations that the support consists only of carbon atoms andthat the support substantially consists of carbon atoms. An element(s)other than carbon atom may be contained. The expression “substantiallyconsists of carbon atoms” denotes that impurities of about 2 to 3 wt %or less can be contaminated.

More preferably, in view of easy formation of a desired pore spaceinside a support, carbon black is used; and particularly preferably,supports manufactured by the method disclosed in JP-A-2010-208887, WO2009/75264, or the like is used.

Besides the aforementioned carbon materials, a porous metal such as Sn(tin) or Ti (titanium) or a conductive metal oxide can also be used asthe support.

A BET specific surface area of a support may be a specific surface areaenough to highly disperse and support a catalyst component thereon. TheBET specific surface area of support is substantially equivalent to theBET specific surface area of catalyst. The BET specific surface area ofsupport is preferably in the range of 1000 to 3000 m²/g, more preferablyin the range of 1100 to 1800 m²/g. If the specific surface area iswithin such a range, since sufficient mesopores and micropores can besecured, enough micropores to transport a gas (lower gas transportresistance) can be secured, and a larger number of the catalyst metalscan be placed (supported) in the mesopores. In addition, an electrolyteand catalyst metals in the catalyst layer can be physically separatedfrom each other (contact between catalyst metals and an electrolyte canbe more effectively suppressed and prevented). Therefore, activity ofthe catalyst metals can be more effectively used. Also, due to existenceof a large number of the micropores and mesopores, the function andeffects by the present invention can be further remarkably exhibited, sothat a catalyst reaction can be more effectively facilitated. Further, abalance between dispersibility of a catalyst component on a catalystsupport and an effective availability of a catalyst component can beappropriately controlled. In addition, the micropores function as a gastransport path, and thus, a three phase boundary with water is moreremarkably formed, so that the catalytic activity can be more improved.

An average particle diameter of a support is preferably in the range of20 to 100 nm. If the average primary particle diameter is within such arange, even in the case where the above-described pore structure isformed in the support, mechanical strength can be maintained, and acatalyst layer can be controlled within an appropriate range. As a valueof the “average particle diameter of a support”, unless otherwise noted,a value calculated as an average value of particle diameters ofparticles observed within several or several tens of fields by usingobservation means such as a scanning electron microscope (SEM) or atransmission electron microscope (TEM) is employed. In addition, the“particle diameter” denotes a maximum distance among distances betweenarbitrary two points on an outline of a particle.

In the present invention, there is no need to use the above-describedgranular porous support, so long as the support has the above-describedpore distributions of micropores and mesopores in the catalyst.

Namely, as the support, a non-porous conductive support, nonwovenfabric, carbon paper, carbon cloth, or the like made of carbon fiberconstituting a gas diffusion layer, or the like may be exemplified. Inthis case, the catalyst can be supported on the non-porous conductivesupport or can be directly attached to the nonwoven fabric, the carbonpaper, the carbon cloth, or the like made of the carbon fiberconstituting the gas diffusion layer of the membrane electrode assembly.

A catalyst metal which can be used in the present invention performscatalysis of electrochemical reaction. As a catalyst metal used for ananode catalyst layer, a well-known catalyst can be used in a similarmanner without particular limitation if the catalyst has catalyticeffects on oxidation reaction of hydrogen. In addition, as a catalystmetal used for a cathode catalyst layer, a well-known catalyst can beused in a similar manner without particular limitation if the catalysthas catalytic effects on reduction reaction of oxygen. Specifically, thecatalyst metal can be selected among metals such as platinum, ruthenium,iridium, rhodium, palladium, osmium, tungsten, lead, iron, copper,silver, chromium, cobalt, nickel, manganese, vanadium, molybdenum,gallium, and aluminum, and alloys thereof.

Among them, in view of improved catalytic activity, poison resistance tocarbon monoxide or the like, heat resistance, or the like, a catalystmetal containing at least platinum is preferably used. Namely, thecatalyst metal preferably is platinum or contains platinum and a metalcomponent other than the platinum, more preferably is platinum or aplatinum-containing alloy. Such a catalyst metal can exhibit highactivity. Although a composition of an alloy depends on a kind of themetal constituting the alloy, a content of platinum may be in the rangeof 30 to 90 atom, and a content of a metal constituting the alloytogether with platinum may be in the range of 10 to 70 atom %. Ingeneral, an alloy is obtained by mixing a metal element with at leastone metal element or non-metal element, and is a general term forsubstances having metallic properties. The structure of the alloyincludes an eutectic alloy which is a mixture where component elementsform separate crystals, an alloy where component elements are completelyfused to form a solid solution, an alloy where component elements form aintermetallic compound or a compound between a metal and a non-metal,and the like, and any one thereof may be employed in the presentapplication. A catalyst metal used in an anode catalyst layer and acatalyst metal used in a cathode catalyst layer can be appropriatelyselected from the aforementioned alloys. In this description, unlessotherwise noted, the description of the catalyst metal for the anodecatalyst layer and the catalyst metal for the cathode catalyst layerhave the same definition. However, the catalyst metal for the anodecatalyst layer and the catalyst metal for the cathode catalyst layer arenot necessarily the same, and the catalyst metals can be appropriatelyselected so that the desired functions described above can be attained.

A shape and size of the catalyst metal (catalyst component) are notparticularly limited, but the shapes and sizes of well-known catalystcomponents may be employed. As the shape, for example, a granular shape,a squamous shape, a laminar shape, or the like may be used, but thegranular shape is preferred. In this case, an average particle diameterof catalyst metals (catalyst metal particles) is not particularlylimited, but it is preferably 3 nm or more, more preferably more than 3and 30 nm or less, particularly preferably more than 3 and 10 nm orless. If the average particle diameter of catalyst metals is 3 nm ormore, the catalyst metals are relatively strongly supported in themesopores, and contact with an electrolyte in a catalyst layer can bemore effectively suppressed and prevented. In addition, the microporesare not blocked by the catalyst metals but remain, and thus, a gastransport path can be more efficiently secured, so that gas transportresistance can be further reduced. In addition, elution due to a changein voltage can be prevented, and temporal degradation in performance canbe also suppressed. Therefore, catalytic activity can be furtherimproved, namely, catalyst reaction can be more efficiently facilitated.On the other hand, if the average particle diameter of the catalystmetal particles is 30 nm or less, the catalyst metals can be supportedinside the mesopores of the supports by a simple method, so that acovering ratio of catalyst metals with an electrolyte can be reduced. Inthe present invention, the “average particle diameter of catalyst metalparticles” can be measured as an average value of a crystallite diameterobtained from a half-value width of a diffraction peak of a catalystmetal component in X-ray diffraction (XRD) spectroscopy or as an averagevalue of a particle diameter of catalyst metal particles examined from atransmission electron microscope (TEM).

In this embodiment, a catalyst content per unit catalyst-coated area(mg/cm²) is not particularly limited so long as a sufficientdispersibility of catalyst on a support and power generation performancecan be obtained. For example, the catalyst content is in the range of0.01 to 1 mg/cm². However, in the case where the catalyst containsplatinum or a platinum-containing alloy, a platinum content per unitcatalyst-coated area is preferably 0.5 mg/cm² or less. The usage ofexpensive noble metal catalyst represented by platinum (Pt) or aplatinum alloy induces an increased cost of a fuel cell. Therefore, itis preferable to reduce the cost by decreasing an amount (platinumcontent) of the expensive platinum to the above-described range. A lowerlimit is not particularly limited so long as power generationperformance can be attained, and for example, the lower limit value is0.01 mg/cm² or more. The content of the platinum is more preferably inthe range of 0.02 to 0.4 mg/cm². In this embodiment, since the activityper catalyst weight can be improved by controlling the pore structure ofthe support, the amount of an expensive catalyst can be reduced.

In this description, an inductively coupled plasma emission spectroscopy(ICP) is used for measurement (determination) of a “content of catalyst(platinum) per unit catalyst-coated area (mg/cm²)”. A method ofobtaining a desired “content of catalyst (platinum) per unitcatalyst-coated area (mg/cm²)” can be easily performed by the personskilled in the art, and the content can be adjusted by controlling aslurry composition (catalyst concentration) and a coated amount.

A supported amount (in some cases, referred to as a support ratio) of acatalyst on a support is preferably in the range of 10 to 80 wt %, morepreferably in the range of 20 to 70 wt %, with respect to a total amountof the catalyst support (that is, the support and the catalyst). Thesupported amount within the aforementioned range is preferable in termsof sufficient dispersibility of a catalyst component on a support,improved power generation performance, economical merit, and catalyticactivity per unit weight.

[Catalyst Layer]

As described above, the catalyst of the present invention can reduce gastransport resistance, so that the catalyst can exhibit a high catalyticactivity and in other words, catalyst reaction can be promoted.Therefore, the catalyst of the present invention can be advantageouslyused for an electrode catalyst layer for fuel cell. Namely, the presentinvention provides an electrode catalyst layer for fuel cell includingthe catalyst and the electrode catalyst according to the presentinvention.

FIG. 3 is a schematic diagram illustrating a relationship between acatalyst and an electrolyte in a catalyst layer according to anembodiment of the present invention. As illustrated in FIG. 3, in thecatalyst layer according to the present invention, although the catalystis coated with the electrolyte 26, the electrolyte 26 does not enter themesopores 24 (and the micropores 25) of the catalyst (support 23).Therefore, although the catalyst metal 22 on the surface of the support23 is in contact with the electrolyte 26, the catalyst metal 22supported in the mesopore 24 is not in contact with the electrolyte 26.The catalyst metal in the mesopore forms three-phase boundary with anoxygen gas and water in a state that the catalyst metal is not incontact with the electrolyte, so that a reaction active area of thecatalyst metals can be secured.

Although the catalyst according to the present invention may existeither in a cathode catalyst layer or an anode catalyst layer, thecatalyst is preferably used in a cathode catalyst layer. As describedabove, although the catalyst according to the present invention is notin contact with the electrolyte, the catalyst can be effectively used byforming three-phase boundary of the catalyst and water. This is becausewater is formed in the cathode catalyst layer.

An electrolyte is not particularly limited, but it is preferably anion-conductive polymer electrolyte. Since the polymer electrolyte servesto transfer protons generated in the vicinity of the catalyst activematerial on a fuel electrode side, the polymer electrolyte is alsoreferred to as a proton conductive polymer.

The polymer electrolyte is not particularly limited, but well-knownknowledge in the art can be appropriately referred to. The polymerelectrolytes are mainly classified into fluorine-based polymerelectrolytes and hydrocarbon-based polymer electrolytes depending on atype of an ion-exchange resin as a constituent material.

As an ion-exchange resin constituting the fluorine-based polymerelectrolyte, for example, perfluorocarbon sulfonic acid based polymerssuch as Nafion (registered trademark, produced by DuPont), Aciplex(registered trademark, produced by Asahi Kasei Co., Ltd.), and Flemion(registered trademark, produced by Asahi Glass Co., Ltd.),perfluorocarbon phosphoric acid based polymers, trifluorostyrenesulfonic acid based polymers, ethylene tetrafluoroethylene-g-styrenesulfonic acid based polymers, ethylene-tetrafluoroethylene copolymers,polyvinylidene fluoride-perfluorocarbon sulfonic acid based polymers,and the like may be exemplified. In terms excellent heat resistance,chemical stability, durability, and mechanical strength, thefluorine-based polymer electrolyte is preferably used, and afluorine-based polymer electrolyte formed of a perfluorocarbon sulfonicacid based polymer is particularly preferably used.

As a hydrocarbon-based electrolyte, sulfonated polyether sulfones(S-PES), sulfonated polyaryl ether ketones, sulfonated polybenzimidazolealkyls, phosphonated polybenzimidazole alkyls, sulfonated polystyrenes,sulfonated polyether ether ketones (S-PEEK), sulfonated polyphenylenes(S-PPP), and the like may be exemplified. In terms of manufacturingadvantages such as inexpensive raw materials, simple manufacturingprocesses, and high selectivity of materials, a hydrocarbon-basedpolymer electrolyte is preferably used. These ion-exchange resins may besingly used, or two or more resins may be used together. In addition,the material is not limited to the above-described material, but anothermaterial may be used.

With respect to the polymer electrolyte which serves to transferprotons, proton conductivity is important. In the case where EW of apolymer electrolyte is too large, ion conductivity with in the entirecatalyst layer would be decreased. Therefore, the catalyst layeraccording to the embodiment preferably includes a polymer electrolytehaving a small EW. Specifically, catalyst layer according to theembodiment preferably includes a polymer electrolyte having an EW of1500 g/eq. or less, more preferably includes a polymer electrolytehaving an EW of 1200 g/eq. or less, and particularly preferably includesa polymer electrolyte having an EW of 1000 g/eq. or less.

On the other hand, in the case where the EW is too small, sincehydrophilicity is too high, water is hard to smoothly move. Due to sucha point of view, the EW of polymer electrolyte is preferably 600 g/eq.or more. The EW (Equivalent Weight) represents an equivalent weight ofan exchange group having proton conductivity. The equivalent weight is adry weight of an ion exchange membrane per 1 eq. of ion exchange group,and is represented in units of “g/eq.”.

It is preferable that the catalyst layer includes two types or more ofpolymer electrolytes having different EWs in a power generation surface,and in this case, among the polymer electrolytes, the polymerelectrolyte having the lowest EW is used in an area where relativehumidity of a gas in a passage is 90% or less. By employing suchmaterial arrangement, resistance is decreased irrespective of a currentdensity area, so that cell performance can be improved. The EW ofpolymer electrolyte used in the area where relative humidity of the gasin a passage is 90% or less, that is, EW of polymer electrolyte havingthe lowest EW is preferably 900 g/eq. or less. By this, theabove-described effects can be further more certainly and moreremarkably attained.

The polymer electrolyte having the lowest EW is preferably used in anarea of which temperature is higher than an average temperature of inletand outlet for cooling water. By this, resistance is decreasedirrespective of a current density area, so that cell performance can befurther improved.

In terms decreased resistance value of a fuel cell system, the polymerelectrolyte having the lowest EW is preferably provided in an areawithin the range of ⅗ or less of the passage length from a gas supplyinlet of at least one of a fuel gas and an oxidant gas.

The catalyst layer according to the embodiment may include, between thecatalyst and the polymer electrolyte, a liquid proton conductingmaterial capable of connecting the catalyst and the polymer electrolytein a proton conductible state. By introducing the liquid protonconducting material, a proton transport path through the liquid protonconducting material is provided between the catalyst and the polymerelectrolyte, so that protons necessary for the power generation can beefficiently transported on the surface of the catalyst. By this,availability of the catalyst is improved, and thus an amount of usedcatalyst can be reduced while maintaining power generation performance.The liquid proton conducting material may be interposed between thecatalyst and the polymer electrolyte. The liquid proton conductingmaterial may be disposed in pores (secondary pores) between poroussupports in a catalyst layer or may be disposed in pores (micropores ormesopores: primary pores) in porous supports.

The liquid proton conducting material is not particularly limited if thematerial has ion conductivity and has a function of forming a protontransport path between the catalyst and the polymer electrolyte.Specifically, water, a protic ionic liquid, an aqueous solution ofperchloric acid, an aqueous solution of nitric acid, an aqueous solutionof formic acid, an aqueous solution of acetic acid, and the like may beexemplified.

In the case of using water as the liquid proton conducting material, thewater can be introduced as the liquid proton conducting material intothe catalyst layer by wetting the catalyst layer with a small amount ofliquid water or a humidified gas before the start of power generation.In addition, water generated through electrochemical reaction during theoperation of a fuel cell may be used as the liquid proton conductingmaterial. Therefore, in a state where a fuel cell starts to be operated,the liquid proton conducting material is not necessarily retained. Forexample, a surface distance between the catalyst and the electrolyte ispreferably set to be a diameter of an oxygen ion constituting a watermolecule, that is, 0.28 nm or more. By maintaining such a distance,water (liquid proton conducting material) can be interposed between thecatalyst and the polymer electrolyte (in the liquid conducting materialretaining portion) while maintaining the non-contact state between thecatalyst and the polymer electrolyte, so that a proton transport pathcan be secured by water therebetween.

In the case of using a material such as an ionic liquid other than wateras the liquid proton conducting material, the ionic liquid, the polymerelectrolyte, and the catalyst are preferably allowed to be dispersed ina solution in the preparation of a catalyst ink. However, the ionicliquid may be added at the time of coating a catalyst layer substratewith a catalyst.

In the catalyst according to the present invention, a total area of thecatalyst which is in contact with the polymer electrolyte is set to besmaller than a total area of the catalyst exposed to the liquidconducting material retaining portion.

Comparison of these areas can be performed, for example, by obtaining amagnitude relationship between capacitance of an electrical double layerformed in a catalyst-polymer electrolyte interface and capacitance of anelectrical double layer formed in a catalyst-liquid proton conductingmaterial interface in a state where the liquid conducting materialretaining portion is filled with the liquid proton conducting material.Namely, since capacitance of an electrical double layer is proportionalto an area of an electrochemically effective interface, if thecapacitance of the electrical double layer formed in thecatalyst-electrolyte interface is smaller than the capacitance of theelectrical double layer formed in the catalyst-liquid proton conductingmaterial interface, a contact area of the catalyst with the electrolyteis smaller than an area thereof exposed to the liquid conductingmaterial retaining portion.

Herein, a measuring method for capacitance of an electrical double layerformed in a catalyst-electrolyte interface and capacitance of anelectrical double layer formed in a catalyst-liquid proton conductingmaterial interface, that is, a magnitude relationship between a contactarea of the catalyst with the electrolyte and a contact area of thecatalyst and the liquid proton conducting material (determination methodfor a magnitude relationship between a contact area of the catalyst andthe electrolyte and an area of the catalyst exposed to the liquidconducting material retaining portion) will be described.

Namely, in the catalyst layer according to the embodiment, the followingfour types of interfaces can contribute as capacitance of electricaldouble layer (Cdl):

-   -   (1) catalyst-polymer electrolyte (C-S)    -   (2) catalyst-liquid proton conducting material (C-L)    -   (3) porous support-polymer electrolyte (Cr-S)    -   (4) porous support-liquid proton conducting material (Cr-L)

As described above, since capacitance of an electrical double layer isproportional to an area of an electrochemically effective interface,Cdl_(C-S) (capacitance of an electrical double layer in acatalyst-polymer electrolyte interface) and Cdl_(C-L) (capacitance of anelectrical double layer in a catalyst-liquid proton conducting materialinterface) may be obtained. Therefore, the contribution of the fourtypes of interfaces to capacitance of an electrical double layer (Cdl)can be identified as follows.

First, for example, under a high humidity condition such as 100% RH andunder a lower humidity condition such as 10% RH or less, eachcapacitance of electrical double layers is measured. As a measurementmethod for the capacitance of electrical double layer, cyclicvoltammetry, electrochemical impedance spectroscopy, or the like may beexemplified. From the comparison, the contribution of the liquid protonconducting material (in this case, “water”), that is, theabove-described contributions (2) and (4) can be identified.

In addition, the contributions to capacitance of an electrical doublelayer can be identified by deactivating a catalyst, for example, in thecase of using Pt as the catalyst, by deactivating the catalyst by supplyCO gas to an electrode to be measured to allow CO to be adsorbed on thesurface of Pt. In this state, as described above, under the highhumidity condition and under the low humidity condition, eachcapacitance of electrical double layers is measured by the same method,and from the comparison, the contributions of the catalyst, that is, theabove-described contributions (1) and (2) can be identified.

By using the above-described method, all the contributions (1) to (4)described above can be identified, the capacitance of the electricaldouble layer in the interface between the catalyst and the polymerelectrolyte and the capacitance of the electrical double layer in theinterface between the catalyst and the liquid proton conducting materialcan be obtained.

Namely, a measurement value (A) in a highly-humidified state can beregarded as capacitance of electrical double layer formed in all theinterfaces (1) to (4), and a measurement value (B) in a lowly-humidifiedstate can be regarded as capacitance of the electrical double layerformed in the interfaces (1) and (3). In addition, a measurement value(C) in a catalyst-deactivated and highly-humidified state can beregarded as capacitance of the electrical double layer formed in theinterfaces (3) and (4), and a measurement value (D) in acatalyst-deactivated and lowly-humidified state can be regarded ascapacitance of the electrical double layer formed in the interface (3).

Therefore, the difference between A and C can be regarded as thecapacitance of the electrical double layer formed in the interfaces (1)and (2), and the difference between B and D can be regarded as thecapacitance of the electrical double layer formed in the interface (1).Next, by calculating the difference between these values, i.e.,(A−C)−(B−D), the capacitance of the electrical double layer formed inthe interface (2) can be obtained. In addition, a contact area of thecatalyst with the polymer electrolyte or an exposed area thereof to theconducting material retaining portion can be obtained by, for example,TEM (transmission electron microscope) tomography besides theabove-described method.

If necessary, the catalyst layer may contain additives of a waterrepellent such as polytetrafluoroethylene, polyhexafluoropropylene, andtetrafluoroethylene-hexafluoropropylene copolymer, a dispersant such asa surfactant, a thickener such as glycerin, ethylene glycol (EG),polyvinyl alcohol (PVA), and propylene glycol (PG), a pore-formingagent, or the like.

A thickness (as a dried thickness) of the catalyst layer is preferablyin the range of 0.05 to 30 μm, more preferably in the range of 1 to 20even more preferably in the range of 2 to 15 The thickness can beapplied to both of the cathode catalyst layer and the anode catalystlayer. However, the thickness of the cathode catalyst layer and thethickness of the anode catalyst layer may be equal to or different fromeach other.

(Method of Manufacturing Catalyst Layer)

Hereinafter, a method for manufacturing the catalyst layer will bedescribed as an exemplary embodiment, but the scope of the presentinvention is not limited to the following embodiment. In addition, allthe conditions for the components and the materials of the catalystlayer are as described above, and thus, the description thereof isomitted.

First, a support (in this description, also referred to as a “poroussupport” or a “conductive porous support”) is prepared. Specifically,the support may be manufactured as described above in the method ofmanufacturing the support. By this, pores having a specific poredistribution (pores including micropores and mesopores, a pore volume ofthe micropore being 0.3 cc/g support or more, and/or a mode radius of apore distribution of the micropores being 0.3 nm or more and less than 1nm) can be formed in the support.

Next, the catalyst is supported on the porous support, so that acatalyst powder is prepared. The supporting of the catalyst on theporous support can be performed by a well-known method. For example, awell-known method such as an impregnation method, a liquid phasereduction supporting method, an evaporation drying method, a colloidadsorption method, a spray pyrolysis method, or reverse micelle(micro-emulsion method) may be used. In order to set an average particlediameter of catalyst metals to be within a desired range, after thecatalyst metals are supported on a support(s), heat treatment may beperformed in a reductive ambience. At this time, a heat treatmenttemperature is preferably in the range of 300 to 1200° C., morepreferably in the range of 500 to 1150° C., particularly preferably inthe range of 700 to 1000° C. A reductive ambience is not particularlylimited, so long as it contributes to grain growth of the catalystmetals. The heat treatment is preferably performed under a mixedambience of a reductive gas and an inert gas. The reductive gas is notparticularly limited, but a hydrogen (H₂) gas is preferred. The inertgas is not particularly limited, helium (He), neon (Ne), argon (Ar),krypton (Kr), xenon (Xe), nitrogen (N₂), and the like can be used. Theinert gas may be used alone or in a form of a mixture of two or moretypes of the gases. A heat treatment time is preferably in the range of0.1 to 2 hours, more preferably in the range of 0.5 to 1.5 hours.

Subsequently, a catalyst ink containing the catalyst powder, polymerelectrolyte, and a solvent is prepared. As the solvent, there is noparticular limitation. A typical solvent used for forming a catalystlayer may be similarly used. Specifically, water such as tap water, purewater, ion-exchanged water, distilled water, cyclohexanol, a loweralcohol having 1 to 4 carbons such as methanol, ethanol, n-propanol,isopropanol, n-butanol, sec-butanol, isobutanol, and tert-butanol,propylene glycol, benzene, toluene, xylene, or the like may be used.Besides, acetic acid butyl alcohol, dimethyl ether, ethylene glycol, orthe like may be used as a solvent. These solvents may be used alone ormay be used in a state of a mixture of two or more solvents.

An amount of solvent for preparing the catalyst ink is not particularlylimited so long as the electrolyte can be completely dissolved.Specifically, a concentration (a solid content) of the catalyst powderand the polymer electrolyte is preferably in the range of 1 to 50 wt %in the electrode catalyst ink, more preferably in the range of about 5to 30 wt %.

In the case of using an additive such as a water repellent, adispersant, a thickener, and a pore-forming agent, the additive may beadded to the catalyst ink. In this case, an added amount of the additiveis not particularly limited so long as it does not interfere with theabove-described effects by the present invention. For example, the addedamount of the additive is preferably in the range of 5 to 20 wt %, withrespect to the total weight of the electrode catalyst ink.

Next, a surface of a substrate is coated with the catalyst ink. A methodof coating the substrate is not particularly limited, but a well-knownmethod may be used. Specifically, a well-known method such as a spray(spray coat) method, a Gulliver printing method, a die coater method, ascreen printing method, or a doctor blade method can be used.

As the substrate coated with the catalyst ink, a solid polymerelectrolyte membrane (electrolyte layer) or a gas diffusion substrate(gas diffusion layer) may be used. In this case, after the catalystlayer is formed on a surface of a solid polymer electrolyte membrane(electrolyte layer) or a gas diffusion substrate (gas diffusion layer),the resultant laminate may be used as it is for manufacturing a membraneelectrode assembly. Alternatively, as the substrate, a peelablesubstrate such as a polytetrafluoroethylene (PTFE) [Teflon (registeredtrademark)] sheet can be used, and after a catalyst layer is formed onthe substrate, the catalyst layer portion can be peeled off from thesubstrate, so that the catalyst layer may be obtained.

Finally, the coat layer (film) of the catalyst ink is dried under an airambience or under an inert gas ambience at a temperature ranging fromroom temperature to 150° C. for a time ranging from 1 to 60 minutes. Bythis, the catalyst layer can be formed.

(Membrane Electrode Assembly)

According to another embodiment of the present invention, provided is amembrane electrode assembly for a fuel cell including theabove-described electrode catalyst layer for fuel cell. Namely, providedis a membrane electrode assembly for fuel cell which comprises a solidpolymer electrolyte membrane 2, a cathode catalyst layer disposed on oneside of the electrolyte membrane, an anode catalyst layer disposed onthe other side of the electrolyte membrane, and a pair of gas diffusionlayers (4 a, 4 c) interposing the electrolyte membrane 2, the anodecatalyst layer 3 a, and the cathode catalyst layer 3 c. In the membraneelectrode assembly, at least one of the cathode catalyst layer and theanode catalyst layer is the catalyst layer according to the embodimentdescribed above.

However, by taking into consideration necessity of improved protonconductivity and improved transport characteristic (gas diffusibility)of a reaction gas (particularly, O₂), at least the cathode catalystlayer is preferably the catalyst layer according to the embodimentdescribed above. However, the catalyst layer according to the embodimentis not particularly limited. The catalyst layer may be used as the anodecatalyst layer or may be used as the cathode catalyst layer and theanode catalyst layer.

According to further embodiment of the present invention, provided is afuel cell including the membrane electrode assembly according to theembodiment. Namely, according to one aspect, the present inventionprovides a fuel cell comprising a pair of anode separator and cathodeseparator interposing the membrane electrode assembly according to theembodiment.

Hereinafter, members of a PEFC 1 using the catalyst layer according tothe embodiment will be described with reference to FIG. 1. However, thepresent invention has features with respect to the catalyst layer.Therefore, among members constituting the fuel cell, specific forms ofmembers other than the catalyst layer may be appropriately modified withreference to well-known knowledge in the art.

(Electrolyte Membrane)

An electrolyte membrane is configured with a solid polymer electrolytemembrane 2 in the same form illustrated in, for example, FIG. 1. Thesolid polymer electrolyte membrane 2 serves to selectively transmitprotons generated in an anode catalyst layer 3 a to a cathode catalystlayer 3 c in the thickness direction during the operation of the PEFC 1.In addition, the solid polymer electrolyte membrane 2 also serves as apartition wall for preventing a fuel gas supplied to an anode side frombeing mixed with an oxidant gas supplied to a cathode side.

An electrolyte material constituting the solid polymer electrolytemembrane 2 is not particularly limited, but well-known knowledge in theart may be appropriately referred to. For example, the fluorine-basedpolymer electrolyte or the hydrocarbon-based polymer electrolytedescribed above as the polymer electrolyte can be used. There is no needto use the polymer electrolyte which is necessarily the same as thepolymer electrolyte used for the catalyst layer.

A thickness of the electrolyte layer is not particularly limited, but itmay be determined by taking into consideration characteristics of theobtained fuel cell. The thickness of the electrolyte layer is typicallyin the range of about 5 to 300 μm. If the thickness of the electrolytelayer is within such a range, balance between strength during the filmformation or durability during the use and output characteristics duringthe use can be appropriately controlled.

(Gas Diffusion Layer)

A gas diffusion layer (anode gas diffusion layer 4 a, cathode gasdiffusion layer 4 c) serves to facilitate diffusion of a gas (fuel gasor oxidant gas) supplied through a gas passage (6 a, 6 c) of a separatorto a catalyst layer (3 a, 3 c) and also serves as an electron conductingpath.

A material constituting a substrate of the gas diffusion layers (4 a, 4c) is not particularly limited, but well-known knowledge in the relatedart may be appropriately referred to. For example, a sheet-shapedmaterial having conductivity and porous property such as a fabric madeof carbon, a sheet-shaped paper, felt, and a nonwoven fabric may beexemplified. A thickness of the substrate may be appropriatelydetermined by considering characteristics of the obtained gas diffusionlayer. The thickness of the substrate may be in the range of about 30 to500 μm. If the thickness of the substrate is within such a range,balance between mechanical strength and diffusibility of gas, water, andthe like can be appropriately controlled.

The gas diffusion layer preferably includes a water repellent for thepurpose of preventing a flooding phenomenon or the like by improvingwater repellent property. The water repellent is not particularlylimited, but fluorine-based polymer materials such aspolytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVdF),polyhexafluoropropylene, and tetrafluoroethylene-hexafluoropropylenecopolymer (FEP), polypropylene, polyethylene, and the like may beexemplified.

In order to further improve water repellent property, the gas diffusionlayer may include a carbon particle layer (microporous layer (MPL), notshown) configured with an assembly of carbon particles including a waterrepellent provided at the catalyst-layer side of the substrate.

Carbon particles included in the carbon particle layer are notparticularly limited, but well-known materials in the art such as carbonblack, graphite, and expandable graphite may be appropriately employed.Among the materials, due to excellent electron conductivity and a largespecific surface area, carbon black such as oil furnace black, channelblack, lamp black, thermal black, and acetylene black can be preferablyused. An average particle diameter of the carbon particles may be set tobe in the range of about 10 to 100 nm. By this, high water-repellentproperty by a capillary force can be obtained, and contacting propertywith the catalyst layer can be improved.

As the water repellent used for the carbon particle layer, theabove-described water repellent may be exemplified. Among the materials,due to excellent water repellent property and excellent corrosionresistance during the electrode reaction, the fluorine-based polymermaterial can be preferably used.

A mixing ratio of the carbon particles and the water repellent in thecarbon particle layer may be set to be in the range of weight ratio ofabout 90:10 to 40:60 (carbon particle: water repellent) by taking intoconsideration balance between water repellent property and electronconductivity. Meanwhile, a thickness of the carbon particle layer is notparticularly limited, but it may be appropriately determined by takinginto consideration water repellent property of the obtained gasdiffusion layer.

(Method of Manufacturing Membrane Electrode Assembly)

A method of manufacturing a membrane electrode assembly is notparticularly limited, and a well-known method in the art may be used.For example, a method which comprises transferring a catalyst layer to asolid polymer electrolyte membrane by using a hot press, or coating asolid polymer electrolyte membrane with a catalyst layer and drying thecoating, and joining the resulting laminate with gas diffusion layers,or a method which comprises coating a microporous layer (in the case ofnot including a microporous layer, one surface of a substrate layer) ofa gas diffusion layer with a catalyst layer in advance and drying theresulting product to produce two gas diffusion electrodes (GDEs), andjoining both surfaces of the solid polymer electrolyte membrane with thetwo gas diffusion electrodes by using a hot press can be used. Thecoating and joining conditions by hot press and the like may beappropriately adjusted according to a type of the polymer electrolyte(perfluorosulfonic acid-based or hydrocarbon-based) in the solid polymerelectrolyte membrane or the catalyst layer.

(Separator)

In the case of configuring a fuel cell stack by connecting a pluralityof unit fuel cells of polymer electrolyte fuel cells in series, aseparator serves to electrically connect the cells in series. Theseparator also serves as a partition wall for separating a fuel gas, anoxidant gas, and a coolant from each other. In order to secure a passagethereof, as described above, gas passages and coolant passages arepreferably installed in each of the separators. As a materialconstituting the separator, well-known materials in the art of carbonsuch as dense carbon graphite and a carbon plate, a metal such as astainless steel, or the like can be employed without limitation. Athickness or size of the separator, a shape or size of the installedpassages, and the like are not particularly limited, but they can beappropriately determined by taking into consideration desired outputcharacteristics and the like of the obtained fuel cell.

A manufacturing method for the fuel cell is not particularly limited,and well-known knowledge in the art in the field of fuel cell may beappropriately referred to.

Furthermore, in order that the fuel cell can generate a desired voltage,a fuel cell stack may be formed by connecting a plurality of membraneelectrode assemblies in series through a separator. A shape and the likeof the fuel cell are not particularly limited, and they may beappropriately determined so as to obtain desired cell characteristicssuch as a voltage.

The above-described PEFC or membrane electrode assembly uses thecatalyst layer having excellent power generation performance andexcellent durability. Therefore, the PEFC or membrane electrode assemblyshows excellent power generation performance and durability.

The PEFC according to the embodiment and the fuel cell stack using thePEFC can be mounted on a vehicle, for example, as a driving powersource.

EXAMPLE

The effects of the present invention will be described with reference tothe following Examples and Comparative Examples. However, the scope ofthe present invention is not limited to the Examples.

Synthesis Example 1

By the followings, a support A having a pore volume of micropores of0.92 cc/g, a pore volume of mesopores of 1.53 cc/g, a mode diameter ofmicropores of 0.8 nm, a mode diameter of mesopores of 2.4 nm, and a BETspecific surface area of 1600 m²/g was prepared. Specifically, thesupport A was prepared by the method disclosed in JP-A-2010-208887 orthe like.

Synthesis Example 2

By the followings, a support B having a pore volume of micropores of1.01 cc/g, a pore volume of mesopores of 1.49 cc/g, a mode diameter ofmicropores of 0.8 nm, a mode diameter of mesopores of 2.4 nm, and a BETspecific surface area of 1600 m²/g was prepared. Specifically, thesupport B was prepared by preparing the support A and pulverizing thesupport A by a bead mill.

Synthesis Example 3

By the followings, a support C having a pore volume of micropores of1.04 cc/g, a pore volume of mesopores of 0.92 cc/g, a mode diameter ofmicropores of 0.65 nm, a mode diameter of mesopores of 1.2 nm, and a BETspecific surface area of 1770 m²/g was prepared. Specifically, thesupport C was prepared by the method disclosed in WO 2009/75264 or thelike.

Example 1

The support A manufactured in the Synthesis Example 1 was used, andplatinum (Pt) having an average particle diameter of more than 3 nm and5 nm or less as the catalyst metal was supported on the support at asupport ratio of 30 wt %, to prepare a catalyst powder A. To bespecific, 46 g of the support A was immersed into 1000 g of adinitrodiammine platinum nitric acid solution having a platinumconcentration of 4.6 wt % (platinum content: 46 g), and after stirring,100 mL of 100% of ethanol as a reducing agent was added thereto. Theresultant mixture was stirred and mixed at a boiling point for 7 hours,so that platinum was supported on the support A. Next, by filtering anddrying, the catalyst powder having a support ratio of 30 wt % wasobtained. After that, the resulting product was maintained in a hydrogenambience at a temperature of 900° C. for 1 hour, to yield a catalystpowder A.

The resultant catalyst powder A was tested for pore volumes ofmicropores and the mesopores, mode diameters of micropores andmesopores, and a BET specific surface area were measured. The resultsare listed in the following Table 1.

Example 2

A catalyst powder B was obtained by the same processes as those ofExample 1, except that the support B manufactured in the SynthesisExample 2 was used instead of the support A in Example 1. The resultantcatalyst powder B was tested for pore volumes of micropores and themesopores, mode diameters of micropores and mesopores, and a BETspecific surface area were measured. The results are listed in thefollowing Table 1.

Example 3

A catalyst powder C was obtained by the same processes as those ofExample 1, except that the support D was used instead of the support Ain Example 1. In this case, the support D is a support (Black Pearls2000 produced by Cabot) having a pore volume of micropores of 0.49 cc/g,a pore volume of mesopores of 0.50 cc/g, a mode diameter of microporesof 0.45 nm, a mode diameter of mesopores of 1.7 nm, and a BET specificsurface area of 1440 m²/g. The resultant catalyst powder C was testedfor pore volumes of micropores and the mesopores, mode diameters ofmicropores and mesopores, and a BET specific surface area were measured.The results are listed in the following Table 1.

Example 4

A catalyst powder D was obtained by the same processes as those ofExample 1, except that the support C manufactured in the SynthesisExample 3 was used instead of the support A in Example 1. The resultantcatalyst powder D was tested for pore volumes of micropores and themesopores, mode diameters of micropores and mesopores, and a BETspecific surface area were measured. The results are listed in thefollowing Table 1.

Comparative Example 1

Ketjen Black EC300J (Ketjen Black International) (support E) was used,and platinum (Pt) having an average particle diameter of 3 to 5 nm asthe catalyst metal was supported on the support at a support ratio of 30wt %, to prepare a comparative catalyst powder F. To be specific, 46 gof the support A was immersed into 1000 g of a dinitrodiammine platinumnitric acid solution having a platinum concentration of 4.6 wt %(platinum content: 46 g), and after stirring, 100 mL of 100% of ethanolas a reducing agent was added thereto. The resultant mixture was stirredand mixed at a boiling point for 7 hours, so that platinum was supportedon the support E. Next, by filtering and drying, a comparative catalystpowder E having a support ratio of 50 wt % was obtained. The resultantcomparative catalyst powder E was tested for pore volumes of microporesand the mesopores, mode diameters of micropores and mesopores, and a BETspecific surface area were measured. The results are listed in thefollowing Table 1. The pore volumes of micropores and mesopores and themode diameters of micropores and mesopores of the support before thesupporting of Pt are also listed in Table 1.

In addition, a pore distribution of the resultant comparative catalystpowder E is shown in FIG. 4. As illustrated in FIG. 4, in thecomparative catalyst powder E, the mode diameters of micropores andmesopores could not be clearly observed. Therefore, in the followingTable 1, for the comparative catalyst powder E, the mode diameters ofmicropores and mesopores of the support (before the supporting of Pt)and of the catalyst (after the supporting of Pt) are indicated by “-”.

Comparative Example 2

A comparative catalyst powder F was obtained by the same processes asthose of Comparative Example 1, except that the graphite Ketjen Black(support F) which had been obtained by calcining the support E (KetjenBlack) in an electric furnace in a nitrogen ambience at a temperature2000° C. for 1 hour was used as the support instead of the support E inComparative Example 1. The resultant comparative catalyst powder F wastested for pore volumes of micropores and the mesopores, mode diametersof micropores and mesopores, and a BET specific surface area weremeasured. The results are listed in the following Table 1. The porevolumes of micropores and mesopores and the mode diameters of microporesand mesopores of the support before the supporting of Pt are also listedin Table 1.

In addition, a pore distribution of the resultant comparative catalystpowder F is shown in FIG. 4. As illustrated in FIG. 4, in thecomparative catalyst powder F, the mode diameters of micropores andmesopores could not be clearly observed. Therefore, in the followingTable 1, for the comparative catalyst powder F, the mode diameters ofmicropores and mesopores of the support (before the supporting of Pt)and of the catalyst (after the supporting of Pt) are indicated by “-”.

Comparative Example 3

A comparative catalyst powder G was obtained by the same processes asthose of Comparative Example 1, except that after supporting platinum onthe support in Comparative Example 1, the resultant support wasmaintained in a hydrogen ambience at a temperature of 900° for 1 hour.The resultant comparative catalyst powder G was tested for pore volumesof micropores and the mesopores, mode diameters of micropores andmesopores, and a BET specific surface area were measured. The resultsare listed in the following Table 1. The pore volumes of micropores andmesopores and the mode diameters of micropores and mesopores of thesupport before the supporting of Pt are also listed in Table 1.

In addition, a pore distribution of the resultant comparative catalystpowder G is shown in FIG. 4. As illustrated in FIG. 4, in thecomparative catalyst powder F, the mode diameters of micropores andmesopores could not be clearly observed. Therefore, in the followingTable 1, for the comparative catalyst powder F, the mode diameters ofmicropores and mesopores of the support (before the supporting of Pt)and of the catalyst (after the supporting of Pt) are indicated by “-”.

Example 5

A catalyst powder H was obtained by the same processes as those ofComparative Example 1, except that the support A was used instead of thesupport E in Comparative Example 1. The resultant catalyst powder H wastested for pore volumes of micropores and the mesopores, mode diametersof micropores and mesopores, and a BET specific surface area weremeasured. The results are listed in the following Table 1. The porevolumes of micropores and mesopores and the mode diameters of microporesand mesopores of the support before the supporting of Pt are also listedin Table 1.

TABLE 1 Support (Before Supporting of Pt) Support (After Supporting ofPt) Micropore Mesopore Micropore Mesopore Pore Mode Pore Mode Pore ModeDecreased Pore Mode BET Specific Volume Radius Volume Radius VolumeRadius Pore Volume Volume Radius Decreased Surface Area (cc/g*¹) (nm)(cc/g*¹) (nm) (cc/g*¹) (nm) (cc/g*¹) (cc/g*¹) (nm) Pore Volume (m²/g*²)Example 1 0.92 0.8 1.53 2.4 0.83 0.8 0.09 1.35 2.1 0.18 1480 Example 21.01 0.8 1.49 2.4 0.87 0.8 0.14 1.28 2.4 0.21 1440 Example 3 0.49 0.450.50 1.7 0.42 0.4 0.07 0.42 2.1 0.08 1210 Example 4 1.04 0.65 0.92 1.21.05  0.65 0.00 0.90 1.2 0.02 1750 Comparative 0.29 — 0.35 — 0.21 — 0.080.29 — 0.06 560 Example 1 Comparative 0.00 — 0.14 — 0.00 — 0.00 0.15 —0.00 180 Example 2 Comparative 0.29 — 0.35 — 0.28 — 0.00 0.36 — −0.01710 Example 3 Example 5 0.92 0.8 1.53 2.4 0.63 0.8 0.29 1.14 2.1 0.41300 *¹Unit of pore volume is cc/g support. *²Unit of BET specificsurface area is m²/g support.

It is noted from the Table 1 that although both of the pore volumes ofmesopores and micropores in the catalyst powders A to D and H accordingto the present invention are decreased, the decreased pore volume ofmesopores is larger. Therefore, it is considered that in the catalystsaccording to the present invention, the catalyst metals are selectivelysupported inside the mesopores. Although it is noted from the Table 1that the pore volumes of micropores in the catalyst powders A to D and Haccording to the present invention are slightly decreased, it isestimated that this is because the catalyst metals block entranceportions of the micropores.

Example 6

The catalyst powder A manufactured in Example 1 and an ionomerdispersion liquid (Nafion (registered trademark) D2020, EW=1100 g/mol,produced by DuPont) as a polymer electrolyte were mixed at a weightratio of the carbon support and the ionomer of 0.9. Next, a cathodecatalyst ink was prepared by adding a n-propyl alcohol solution (50%) asa solvent with a solid content (Pt+carbon support+ionomer) of 7 wt %.

Ketjen Black (particle diameter: 30 to 60 nm) was used as a support, andplatinum (Pt) having an average particle diameter of 2.5 nm as thecatalyst metal was supported thereon at a support ratio of 50 wt %, toobtain a catalyst powder. The catalyst powder and an ionomer dispersionliquid (Nafion (registered trademark) D2020, EW=1100 g/mol, produced byDuPont) as the polymer electrolyte were mixed at a weight ratio of thecarbon support and the ionomer of 0.9. Next, an anode catalyst ink wasprepared by adding a n-propyl alcohol solution (50%) as a solvent with asolid content (Pt+carbon support+ionomer) of 7 wt %.

Next, a gasket (Teonex produced by Teijin DuPont, thickness: 25 μm(adhesive layer: 10 μm)) was arranged around both surfaces of a polymerelectrolyte membrane (NAFION NR211 produced by DuPont, thickness: 25μm). Then, an exposed portion of one surface of the polymer electrolytemembrane was coated with the catalyst ink having a size of 5 cm×2 cm bya spray coating method. The catalyst ink was dried by maintaining thestage where the spray coating was performed at a temperature of 60° C.for 1 minute, to obtain an electrode catalyst layer. At this time, asupported amount of platinum is 0.15 mg/cm². Next, similarly to thecathode catalyst layer, an anode catalyst layer was formed by spraycoating and heat-treatment on the electrolyte membrane, to obtain amembrane electrode assembly (1) (MEA (1)) of this example.

Example 7

A membrane electrode assembly (2) (MEA (2)) was manufactured by the sameprocesses as those of Example 6, except that the catalyst powder Bobtained in Example 2 was used instead of the catalyst powder A inExample 6.

Example 8

A membrane electrode assembly (3) (MEA (3)) was manufactured by the sameprocesses as those of Example 6, except that the catalyst powder Cobtained in Example 3 was used instead of the catalyst powder A inExample 6.

Example 9

A membrane electrode assembly (4) (MEA (4)) was manufactured by the sameprocesses as those of Example 6, except that the catalyst powder Dobtained in Example 4 was used instead of the catalyst powder A inExample 6.

Comparative Example 4

A comparative membrane electrode assembly (1) (comparative MEA (1)) wasmanufactured by the same processes as those of Example 6, except thatthe comparative catalyst powder E obtained in Comparative Example 1 wasused instead of the catalyst powder A in Example 6.

Comparative Example 5

A comparative membrane electrode assembly (2) (comparative MEA (2)) wasmanufactured by the same processes as those of Example 6, except thatthe comparative catalyst powder F obtained in Comparative Example 2 wasused instead of the catalyst powder A in Example 6.

Comparative Example 6

A comparative membrane electrode assembly (3) (comparative MEA (3)) wasmanufactured by the same processes as those of Example 6, except thatthe comparative catalyst powder G obtained in Comparative Example 3 wasused instead of the catalyst powder A in Example 6.

Example 10

A membrane electrode assembly (5) (MEA (5)) was manufactured by the sameprocesses as those of Example 6, except that the catalyst powder Hobtained in Example 5 was used instead of the catalyst powder A inExample 6.

Experiment 1 Evaluation of Oxygen Reduction Reaction (ORR) Activity

The membrane electrode assemblies (1) to (5) manufactured in Examples 6to 10 and the comparative membrane electrode assemblies (1) to (3)manufactured in Comparative Examples 4 to 6 were evaluated for oxygenreduction reaction activity by measuring power generation current persurface area of platinum (μA/cm² (Pt)) at 0.9 V under the followingevaluation conditions. The results are shown in the following Table 2.

[Chem. 2]

<Evaluation Conditions>

Temperature: 80° C.

Gas Component: Hydrogen (Anode Side)/Oxygen (Cathode Side)

Relative Humidity: 100% RH/100% RH

Pressure: 150 kPa(abs)/150 kPa(abs)

Voltage Scan Direction: Anode

Experiment 2 Evaluation of Power Generation Performance

The membrane electrode assemblies (1) to (5) manufactured in Examples 6to 10 and the comparative membrane electrode assemblies (1) to (3)manufactured in Comparative Examples 4 to 6 were evaluated for powergeneration performance by measuring voltage (V) at 1.5 A/cm² under thefollowing evaluation conditions. The results are shown in the followingTable 2.

[Chem. 3]

<Evaluation Conditions>

Temperature: 80° C.

Gas Component: Hydrogen (Anode Side)/Nitrogen (Cathode Side)

Relative Humidity: 90% RH/90% RH

Pressure: 200 kPa(abs)/200 kPa(abs)

Experiment 3 Evaluation of Gas Transport Resistance

The membrane electrode assemblies (1) to (5) manufactured in Examples 6to 10 and the comparative membrane electrode assemblies (1) to (3)manufactured in Comparative Examples 4 to 6 were evaluated for gastransport resistance in accordance with the method disclosed in T.Mashio et al. ECS Trans. 11, 529, (2007). The results are shown in thefollowing Table 2.

To be specific, a limiting current density (A/cm²) was measured withdilute oxygen. At this time, gas transport resistance (s/m) wascalculated from a slope of the limiting current density (A/cm²) relativeto a partial pressure (kPa) of oxygen. Since the gas transportresistance is proportional to a total pressure of the gas, the gastransport resistance can be divided into a component depending on thetotal pressure of the gas (gas transport resistance by diffusion ofmolecules) and a component not depending on the total pressure of thegas. Among these, the former is a transport resistance component inpores having such a relatively large size as 100 nm or more existing inthe gas diffusion layer or the like, and the latter is a transportresistance component in pores having such a relatively small size asless than 100 nm existing in the catalyst layer or the like. In thismanner, the total pressure dependency of gas transport resistance wasmeasured, and the component not depending on the total pressure wasextracted, so that the gas transport resistance in the catalyst layerwas obtained.

TABLE 2 Oxygen Reduction Reaction Activity Gas Trans- Power GenerationPower Generation portability Current (μA/cm² (Pt)) Performance GasTransport per Surface Area Voltage at 1.5 Resistance of Platinum at 0.9V A/cm² (V) (s/m) Example 6 932 0.604 3.6 Example 7 820 0.611 6.4Example 8 813 0.594 7.9 Example 9 828 0.671 1.1 Comparative 339 0.46620.4 Example 4 Comparative 378 0.347 16.0 Example 5 Comparative 5810.482 16.4 Example 6 Example 10 430 0.578 10.3

It is noted from the Table 2 that the MEAs (1) to (5) using the catalystaccording to the present invention have a significantly lower gastransport resistance and more excellent catalyst activity (oxygenreduction reaction activity) and power generation performance incomparison with the MEAs (1) to (3) which do not have the poredistribution defined by the present invention. It is considered from theresults in the Tables 1 and 2 that the catalyst according to the presentinvention has improved gas transportability and the catalyst metalsinside the mesopores can form three phase boundary with water in anuncontacted state with the electrolyte, so that a high catalyst activitycan be exhibited.

The present application is based on the Japanese Patent Application No.2013-092911 filed on Apr. 25, 2013, the entire disclosed contents ofwhich are incorporated herein by reference.

DESCRIPTION OF REFERENCE SIGNS

-   1 Polymer electrolyte fuel cell (PEFC),-   2 Solid polymer electrolyte membrane,-   3 Catalyst layer,-   3 a Anode catalyst layer,-   3 c Cathode catalyst layer,-   4 a Anode gas diffusion layer,-   4 c Cathode gas diffusion layer,-   5 Separator,-   5 a Anode separator,-   5 c Cathode separator,-   6 a Anode gas passage,-   6 c Cathode gas passage,-   7 Coolant passage,-   10 Membrane electrode assembly (MEA),-   20 Catalyst,-   22 Catalyst metal,-   23 Support,-   24 Mesopore,-   25 Micropore,-   26 Electrolyte.

The invention claimed is:
 1. An electrode catalyst layer for fuel cell comprising a catalyst and an electrolyte, wherein the catalyst comprises a catalyst support and a catalyst metal supported on the catalyst support, wherein the catalyst includes pores having a radius of less than 1 nm and pores having a radius of 1 nm or more, wherein a pore volume of the pores having a radius of less than 1 nm is 0.3 to 2.0 cc/g support, wherein the catalyst metal is supported inside the pores having a radius of 1 nm or more, wherein a larger number of the catalyst metal is supported in the pores having a radius of 1 nm or more than in the pores having a radius of less than 1 nm, and wherein a mode radius of a pore distribution of the pores having a radius of 1 nm or more of the catalyst is in the range of 1 to 5 nm.
 2. The electrode catalyst layer for fuel cell according to claim 1, wherein an average particle diameter of the catalyst metal is 3 nm or more.
 3. The electrode catalyst layer for fuel cell according to claim 1, wherein a pore volume of the pores having a radius of 1 nm or more is 0.4 cc/g support or more.
 4. The electrode catalyst layer for fuel cell according to claim 1, wherein a BET specific surface area of the catalyst is 1440 m²/g support or more.
 5. The electrode catalyst layer for fuel cell according to claim 1, wherein the catalyst metal is platinum or includes platinum and a metal component other than platinum.
 6. A membrane electrode assembly for fuel cell comprising the electrode catalyst layer for fuel cell set forth in claim
 1. 7. A fuel cell comprising the membrane electrode assembly for fuel cell set forth in claim
 6. 8. An electrode catalyst layer for fuel cell comprising a catalyst and an electrolyte, wherein the catalyst comprises a catalyst support and a catalyst metal supported on the catalyst support, wherein the catalyst includes pores having a radius of less than 1 nm and pores having a radius of 1 nm or more, wherein a mode radius of a pore distribution of the pores having a radius of less than 1 nm is 0.3 nm or more and 0.8 nm or less, and wherein the catalyst metal is supported inside the pores having a radius of 1 nm or more, wherein a larger number of the catalyst metal is supported in the pores having a radius of 1 nm or more than in the pores having a radius of less than 1 nm.
 9. The electrode catalyst layer for fuel cell according to claim 8, wherein an average particle diameter of the catalyst metal is 3 nm or more.
 10. The electrode catalyst layer for fuel cell according to claim 8, wherein a pore volume of the pores having a radius of 1 nm or more is 0.4 cc/g support or more.
 11. The electrode catalyst layer for fuel cell according to claim 8, wherein a mode radius of a pore distribution of the pores having a radius of 1 nm or more of the catalyst is in the range of 1 to 5 nm.
 12. The electrode catalyst layer for fuel cell according to claim 8, wherein a BET specific surface area of the catalyst is 1440 m²/g support or more.
 13. The electrode catalyst layer for fuel cell according to claim 8, wherein the catalyst metal is platinum or includes platinum and a metal component other than platinum.
 14. The electrode catalyst layer for fuel cell according to claim 8, wherein a pore volume of the pores having a radius of less than 1 nm is 0.3 cc/g support or more and 2.0 cc/g support or less.
 15. A membrane electrode assembly for fuel cell comprising the electrode catalyst layer for fuel cell set forth in claim
 8. 16. A fuel cell comprising the membrane electrode assembly for fuel cell set forth in claim
 15. 17. The electrode catalyst layer for fuel cell according to claim 1, wherein the pore volume of the pores having a radius of less than 1 nm is 0.83 to 2.0 cc/g support.
 18. The electrode catalyst layer for fuel cell according to claim 8, wherein the mode radius of the pore distribution of the pores having a radius of less than 1 nm is 0.65 nm or more and 0.8 nm or less.
 19. An electrode catalyst layer for fuel cell comprising a catalyst and an electrolyte, wherein the catalyst comprises a catalyst support and a catalyst metal supported on the catalyst support, wherein the catalyst includes pores having a radius of less than 1 nm and pores having a radius of 1 nm or more, wherein a pore volume of the pores having a radius of less than 1 nm is 0.3 to 2.0 cc/g support, wherein the catalyst metal is supported inside the pores having a radius of 1 nm or more, wherein a larger number of the catalyst metal is supported in the pores having a radius of 1 nm or more than in the pores having a radius of less than 1 nm, and wherein a BET specific surface area of the catalyst is 1440 m²/g support or more. 